23/5000 Add firebase data

Ionic Framework ionic-v3
1

Hello I tried to do the sum and I sale NaN, somebody can help me add the prices price with firebase

export class AboutPage {
  meVendi: FirebaseListObservable<any>;

    constructor(public navCtrl: NavController,public alertCtrl:AlertController, public database:AngularFireDatabase, public event:Events ) {
    this.meVendi = this.database.list('/Evendido');

  }
  total_suma = 0;

  suma (valor){
    this.total_suma+=valor;
    
  }
  

 


}
  

<ion-content padding>
  <ion-list>
    <ion-item-sliding *ngFor="let item of meVendi|async" >
      <ion-item>
        <ion-label>
          <h1>Carlos Pidio</h1>
         
            <ion-list>
              <h2>{{item.Platito.name}} &nbsp;&nbsp;${{item.Platito.price}} {{suma(item.Platito.price)}}</h2>
              <h2>{{item.carnita.name}} &nbsp;&nbsp;${{item.carnita.price}} {{suma(item.carnita.price)}}</h2>
              <h2>{{item.Adicionar.name}} &nbsp;&nbsp;${{item.Adicionar.price}}  {{suma(item.Adicionar.price)}}</h2>
            </ion-list>  
        </ion-label>
      </ion-item>


   
    </ion-item-sliding>
          <span>total dela suma es: {{total_suma}}</span>
    

  </ion-list>

</ion-content>